Isomer distribution during methyl fucoside, formation by the fischer method. Further support for the previously proposed reaction mechanism

Abstract

The formation of the four methyl L-fucosides by the Fischer reaction in boiling methanol, with a strongly acidic ion-exchange resin as catalyst, was followed by gas-liquid chromatography of their trimethylsilyl ethers. The initial rapid formation of all four methyl fucosides differs markedly and predictably from the behavior Of D-galactose, which is enantiomeric with L-fucose except for the presence of the hydroxyl group at C-6 in the latter. It also differs somewhat from the behavior of L-arabinose, which is enantiomeric except for the absence of the methyl group at C-5. The initial boiling methanol solution may contain 5% of an L-fucofuranose, in addition to 38% of α- L-fucopyranose and 57% of β- L-fucopyranose. The final equilibrium mixture contains ≈6% of methyl α- L-fucofuranoside, 13% of β- L-fucofuranoside, 54% of methyl α- L-fucopyranoside, and 27% of methyl β- L-fucopyranoside.